Speaking to reporters just hours ahead of the launch, David Lagercrantz said he was “terrified” as he wrote The Girl in the Spider's Web.
“I used to say that I was bipolar, manic depressive all the time, and I think it was kind of a good thing to write [in this condition]”, he said of the 500-page thriller which picks up the trail of tattooed computer hacker Lisbeth Salander and journalist Mikael Blomkvist.
The sequel went on sale in 25 countries on Thursday, including Sweden where a Stockholm bookstore opened at midnight to sell the first copies to around 50 fans who showed up to get their books signed by the author.
“I came on the subway, that way I can start reading on my way home,” said Millennium fan Rickard de Boussard, 57, who had joined fans queuing outside a branch of Akademibokhandeln in the city centre.
On Wednesday it emerged that a newspaper kiosk at Stockholm's central station had briefly put the book on sale a day early, before being ordered by the book's publishers to remove it.
A total of 2.7 million copies have been printed, including 500,000 in the US.
Fans queuing at midnight in Stockholm. Photo: Henrik Montgomery/TT
Lagercrantz meanwhile told newswire AFP in an interview that he was “obsessive” during the writing process, poring over Larsson's trilogy, reading and re-reading it, doing endless hours of research, questioning his ability the whole time.
“The writing process was a combination of an enormous desire and total fear,” he says with a laugh.
“The fear of not doing Stieg Larsson justice kept me going.
“I was not the easiest person to live with because I was thinking about it all the time,” he told AFP, saying he was “scared to death” that his book would not live up to the trilogy written by Larsson, who died suddenly of a heart attack in 2004 at age 50, before the series gained global fame.
Larsson's three books, published in 2005-2007, have sold 80 million copies worldwide and inspired a series of films in Swedish as well as a Hollywood version. He is the best selling Swedish adult fiction writer of all time.
While many fans craved a fourth instalment, some are not happy — among them Eva Gabrielsson, Larsson's partner for 32 years until his death.
The couple were not married and Larsson left no will, so his estate went to his brother and father.
Gabrielsson, 61, lost a bitter battle with them to manage his work.
She has criticised both the decision to continue the trilogy and to pick Lagercrantz as author, calling him “a totally idiotic choice” in an AFP interview in March.
Larsson had no plans for a continuation of Mikael's and Lisbeth's adventures, and Lagercrantz lacked his left-wing activist background, knowing nothing of the milieu described in the books, she said.
“They say heroes are supposed to live forever. That's a load of crap, this is about money,” Gabrielsson said.
But publishing house Norstedts defended Lagercrantz, a journalist from Stockholm's intelligentsia who penned football star Zlatan Ibrahimovic's official biography, saying he had a “special talent for depicting the world of others.”
“It's my novel in his universe,” Lagercrantz told AFP.
“His world, his characters, but I put some of me in it too.”
Larsson's father and brother say the book's royalties will go to the anti-racist magazine Expo co-founded by the late writer.
The pair are among the few who have already read the thriller, which they lavishly praised.
“I kept it on my nightstand for a week before I opened it. I was a little afraid. But once I started, it was impossible to stop,” Larsson's brother Joakim told AFP.
The writing was shrouded in secrecy with the author, editors and translators all working on computers disconnected from the Internet to prevent hackers from leaking the plot.